Control system with droop correction



April 22. 1952 E. T. DAVIS ET AL CONTROL SYSTEM WITH DROOP CORRECTION 2SHEETSSHEET 1 Filed Jan. 28, 1949 28 Tlj INVENTOR. ELWOOD T. DAVISWILLIAM CLARK JR. BY aw/fizz ATTORNEYS April 1952 E. T. DAVIS ET CONTROLSYSTEM WITH DROOP CORRECTION 2 SHEETSSH Filed Jan. 28, 1949 ,confrolpoint 80 M% ATTQRNEYS Patented Apr. 22, 1952 CONTROL SYSTEM WITH DROOPCORRECTION Elwood '1. Davis, Brookline, and William Clark, In,Philadelphia, Pa., assignors to Leeds and Northrup Company,Philadelphia, Pa., a corporation of Pennsylvania Application January 28,1949, Serial No. 73,384

16 Claims.

This invention relates to systems for varying the application of anagent to control the magnitude of a condition, such as temperature, ionconcentration, pressure, or other physical, chemical or electricalcondition, and has for an object the provision of means for variablypredetermining the extent of adjustment of droop-correcting devicesregardless of the total adjustment of the condition-changing means.

The present invention is an improvement upon the control system of DavisPatent 2,300,537 in which there is disclosed the use of electricallyheated resistors in a ba-lanceable network arranged to predetermine theproportional action of the system within a predetermined throttlingrange. The system also included droop-corrector slidewires operable fromone limit to the other in accordance with the direction and extent ofunbalance of the balanceable network. While such operation of thedroop-correcting slidewires is satisfactory for some applications, ithas been found that additional and valuable control functions can besecured by providing means for limiting the extent of adjustment of thedroop-corrector slidewires for any given adjustment of thecondition-changing means.

In a preferred form of the invention, the droopcorrector slidewires ortheir equivalent are not only driven by a. motor, as indicated in theaforesaid Davis Patent 2,300,537, but they are also manually adjustableindependently of motor position, and an adjustable stop is provided forlimiting as may be desired the extent of adjustment of thedroop-corrector slidewires by the driving motor. For a more detaileddescription of the invention and for further objects and advantagesthereof, reference is to be had to the following description taken inconjunction with the accom panying drawings, in which:

Fig. 1 diagrammatically illustrates a control systerm in which there isshown in exploded perspective the adjusting means and mechanical stopfor-the droop-corrector slidewires; Figs. 2 and 3 are explanatorydiagrams; and Fig. 4 is a sectional view of the assembled adjustingmeans.

Inasmuch as the present invention has been shown as applied to thecontrol system of Fig. l of the aforesaid Davis Patent 2,300,537, there2 have been applied to Fig. l of the present drawings the same referencecharacters used in the aforesaid patent to identify certain like parts,as in Fig. 1 of those drawings. Before describing in detail the manualadjustment and mechanical stop for adjustably predetermining andlimiting the degree of adjustment of the droopcorrector slidewires 29and 30, a brief review of the operation of the system as a whole will bepresented as necessary background for an understanding of the presentinvention.

Though the invention, as aforementioned, is applicable to the control ofmany conditions, the system of Fig. 1 has been illustrated as applied tothe control of the temperature of a furnace F by means of a fuel valve Vin fuel line 5 which valve thus is an element for controllingapplication of the condition-controlling agent. In such case, athermocouple is a suitable condition-responsive element; specifically athermocouple T is subjected to the temperature of the furnace F toproduce a voltage which varies with furnace temperature. Thethermocouple 'I is connected in a balanceable network so that thevoltage thereof is balanced against the effective voltage of apotentiometer slidewire 6 powered by a battery ll through an adjustableresistor Hi. When there is an unbalance voltage as between thethermocouple T and the potentiometer slidewire 6 the galvanometer G willswing its pointer l3 to the right or the left between a pair of feelersI1, I! periodically movable under the influence of a spring It to engagethe pointer [3. As soon as one feeler ll engages the pointer, the

, other feeler l1 angularly moves or positions a clutch member 20relative to a clutch disc 22 for subsequent drive of disc 22, throughthe medium of suitable cams (not shown), to rotate the disc 8, carryingslidewire 5, for rotation of slidewire 6, relative to contact I, in adirection torebalance the network. The angular adjustment by themechanical relay MB of the clutch disc 22 and shaft 9 at the same timerotates the supporting disc 10 of slidewire l of a second ba1- anoeablenetwork N. I

It will be convenient now to assume that the parts in Fig. 1 are inpositions corresponding to a temperature of furnace F at the controlpoint, and further, that the described operation of the mechanical relayMR will occur due to a decrease in the temperature of the furnace F.With a decrease in temperature the voltage of the thermocouple T willdecrease and, hence, will be less than that derived from thepotentiometer 6. Accordingly, the mechanical relay MR will operate thedisc 8 in a clockwise direction. Similarly, the disc Ill will be rotatedin a clockwise direction to reduce the resistance between the contact 2and the terminal L of slidewire I. The character L refers to theterminal of slidewire l corresponding with a temperature lower than thecontrol point, and the character H refers to the other terminal ofslidewire 1 corresponding with a temperature higher than the controlpoint.

The described clockwise movement of disc is and of slidewire Iunbalances the network N. A resulting unbalance voltage appears betweenthe contact 2 of slidewire l and the contact 4' ofslidewire 3 carried bya disc 36 and is applied to a relay R. The relay R is thereby actuatedto complete a circuit from a suitable source of supply exemplified bysupply line conductors 35, 35, as through stationary contact 34, forenerglzation of motor M in a direction to move the valve V toward itsopen position. At the same time thelmotor' M, as indicated by driveconnection 3 I, rotates the disc 36 to move the slidewire 3 in acounterclockwise direction which is in the direction for reba lance ofthe network N. Such counter-clockwise movement is in the directioncorresponding to movement of valve V toward its open position.

If only the effect of adjustment of slidewire 3 be now considered, itwill, of course, be under: stood that as soon as the motor'M has movedthe disc 36 and slidewire 3 an adequate distance, the network will againbe balanced, causing the relay R to move its contact 32 to its openmidposition. The opening of valve V represents a i.

control action tending toreturnthe temperature to its. control point.But without other provisions'included-in' the'system that control actionwouldhave a drooping characteristic; which for many cases isundesirable. the drooping characteristic there are provided thed'ro'op-c rrector slidewires 29- and 30, respectively'inounted ondiscs-28d and 30a. While such drdop-correcting' slidewires. or resistorsmay alone beincludedin networkN', it is'generally desirable alsot'o'include circuit elements which provide an intermittentaction; themagnitude of which isproportional. to. the deviation of the" temperaturefrom the control point.

These additional provisions include temperature-sensitive resistors 31and 38 forming additional arms inthe network N, and heating resistors 39and 42 respectively in heat exchange relation with resistors- 31. and38. Upon adjustment of the slidewire l, as previously described, anunbalance voltage appears between the contact 2 andthe juncture ofresistors 31 and 3B. In this connection-it is assumed that: resistors 24and 25 are of equal value and that resistors 31 and 38 are also of equalvalue at a like temperature.

The unbalance voltage is applied toa relay R1- simultaneously with theapplication of the earlier described unbalance voltage to the relay R.Accordingly, the relay R1 is energized to move its contact 4| intoengagement with stationary contact 43 to energize through a circuitincluding adjustable resistor 45 the heating resistor 42. The heatingresistor 42 elevates the temperature of the resistor 38 to increase its;resistance and, thus, introduces into the network an efiect tend- To.compensate for ing to rebalance that part of the network N comprisingthe two upper branches. The rebalancing efiect upon the upper twobranches, which functionally comprise a balanceable network, tends todeenergize relay R1.

While relay R1 is energized, the closure of the circuit from a supplyline 23, 28 through contact All and stationary contact 23 serves toenergize motor M1 to drive discs 29a and 30a in a clockwise direction tointroduce additional resistance in the arm of the network including orcomprising slidewire 29 and to decrease resistance in the branchincluding or comprising slidewire 30. Accordingly, both slidewires 29and 30 act in directions to unbalance network N in the same direction asthat produced by the adjustment of slidewire disc it by the mechanicalrelay MR. The introduction of the additional unbalanclng effect results,with the temperature of the furnace F at the control point, in a finalposition of valve V, at subsequent balance of network N, at a positionnearer its fully open position than its previous position. Thus there iscompensation for the drooping characteristic above referred to.

If the departure in the temperature of furnace F were small, it will beunderstood that'theheating resistor 152 will not elevate the temperatureof resistor 38 to any great degree. Howevenfor' a substantial departure,the temperature of resis-' tor 38 will be changed considerably toincrease the resistance in its leg or branchof the network.

Since the resistance change is in a rebalanci'ng.

direction, relay R1 will be deenergized before the temperature offurnace F is returned to'the con-'- trol point. The heater coil 42 andits associated resistor 38 thereupon begin to cool. The decrease inresistance of resistor 38 unbalances the network comprising the uppertwo branches, and relay R1 will again close. There will again beanadjustment of slidewires 29 and 39' and this intermittent actioncontinues until the temperature of furnace F is again at the controlpoint.

If the direction of unbalance reverses, relay'Rr will operate to close acircuit through its stationary contact 40 to energize the motorMi in thereverse direction for counterclockwise rotation ofdiscs 29a and 30a. ofslidewires 2'9 and 30.. Such action is in a direction to unbalance thenetwork N to energize the relay R, whil'e ener gization of the heatingresistor 39 initiates elevation of temperature of resistor 31 toincrease its resistance and thus tend again to rebalan'c'ethe network todeenergize relay R1. The described control actions ordinarily take placewith departure of the temperature of the furnace F from. the controlpoint and result in the val-veV', being; adjusted to a new position tosupply fuel, as re quired, to maintain the temperature of the furnace Fat the control point. It is tobe understood that a rise in temperatureresults in control actions like those described but in reverse order.

With the foregoing general understanding of the operation of the systemof Fig. l, more detailed consideration will now be given as to whathappens upon a departure in thetemperature of furnace F from apredetermined, selected value, generally known as the controlpoint,--particularly as regards the return of the temperature to thecontrol point in terms of the initiation. of the droop-correctiveaction. More particularly, provision has been made to shift thethrottling range by means of the droop-correctiveaction with respect tothe control point. Stated more fully, the throttling range may be soshifted that any proportion thereof may be above or below the controlpoint.

In accordance with the present invention a limitation of the aforesaiddescribed shift in the throttling range or proportional band isaccomplished by a combined manually adjustable mechanical sto and by afriction drive 50 between the motor M1 and the slidewire discs 29a and30a.

Though any suitable friction drive 50 may be provided, it has been shownas comprising a clutch member 5011 splined tothe shaft 52 and pressed bya spring 5! against a combined worm wheel and clutch element 50b. On theshaft 52 are mounted the discs 29a and 30a of slidewires 29 and 30. Onthe end of shaft 52 is secured an operating knob 53 having a projection53a extending toward a pointer disc 54 for engagement with a tongue orabutment 54a, extending toward the knob 53. The pointer 54b extendsradially from the disc 5% and projects through a slot 55a of a knurledoperating knob 55, having secured thereto a disc 55b. The disc 55b isprovided with outturned tongues or leaves 550. The knob 55 is hollow orcylindrical and is arranged to receive therein the pointer disc 54 aswell as the cylindrical portion of the knob 53.

In assembled position, Fig. 4, the disc 55b bears against a planesurface of the frame or housing wall 55. A washer-like ring 51 encirclesthe knurled knob 55 but is disposed behind the dial plate 53 and has aradial width great enough to cover the tongues or leaves 550. Thesetongues or leaves 550 are of spring or elastic material and thus resistflattening during assembly of the parts. leaves 550 is determined byspacers 59, Fig. 1, interposed between the surface 56 and the back ofthe dial plate 58, mounting of assembly screws on being provided toextend through openings in the cover plate 58 into threaded openings inthe surface 55 (not shown).

Assuming the parts in Fig. l to be in their assembled positions, it willbe seen that the knob 53 may be rotated in a counter-clockwise directiondirectly to drive the shaft 52 to rotate the slidewires 29 and 30, thisrotation being against the frictional resistance of the friction drive50. 'By a suitable fixed end stop mechanism comprising members 93 and94, Fig. 4, the allowable counter-clockwise rotation of the shaft 52corresponds with maximum movement of slidewires 29 and 30 for inclusionof slidewire 30 in its circuit and for exclusion of slidewire 29 fromits circuit. This limit of movement corresponds with the 0 marking onthe scale of dial plate 58. The fixed end stop mechanism comprisingmembars 93 and 94, Fig. 4, also determines the maximum limit of movementin the opposite direction to correspond with the 100 marking on thescale of the plate 58. It will be noted in Fig. 4 that stop member 94may engage either side of stop member 93 depending upon the direction ofrotation of the slidewides 29 and 30 thus to define their extreme limitsof movement.

If the knob 53 is rotated in a clockwise direction with the partsassembled and angularly disposed as shown in Fig. 1, the projection 53awill strike against the end of tongue or abutment 54a. Further clockwiserotation of knob 53 and shaft 52 is strongly resisted by frictiondeveloped between the spring leaves 55c, the surface 56 and the ring 51.The frictional opposition is made large enough so that an operatorreceives the impression that he has moved the knob 53 against a solidstop. In this connection, it is'to be re- The extent of compression ofthe spring 6 membered that the initial turning of the knob 53 is againstthe frictional opposition of the drive 50 and, thus, upon engagement ofabutment 54a by projection 53a there is added the frictional oppositiondeveloped by or within the assembly associated with the knob 53.

The motor Ml if energized in one direction or the other will also tendto drive the shaft 52 clockwise or counter-clockwise. The motor will beeffective to drive the shaft 52 in a counterclockwise direction, throughits normal range of movement, limited by engagement of end stops 93 and94, but if it rotates in a clockwise direction and drives the projection53a against the end of tongue 54a, the friction developed by theassembly associated with the knob is too great to be overcome by thetorque which may be transmit-- ted through the friction drive 50.Accordingly, the motor M1 cannot adjust the slidewires 29 and 3!! beyondthat position determined by the setting of the knurled knob 55 as shownby the pointer 54b on the scale of plate 58.

In one embodiment of the invention the friction developed in theassembly, including the spring leaves 550, was of the order of fortyounce-inches while the friction drive 58 was adjusted to yield a torqueof thirty ounce-inches. Thus, with a total torque of seventyounce-inches on the knob 53 the impression is created of movementagainst a stationary stop.

In explanation of the new and novel cooperation of the describedarrangement in connection with the system with which it is associated,it will be assumed that the pointer 54b has been set opposite 50 on thedial plate 58. With that setting and with the temperature of the furnaceF at the control point, the valve V will be in its mid-position; thatis, midway of its extreme limits of adjustment. It will be now assumedthat a new and cold charge has been placed in the furnace F. Thetemperature of the furnace F will rapidly decrease and the valve V willbe moved to its fully open position. As the valve V the same time asimilar rider 59 is moved by a: threaded member iii to engage a crank IIto open a limit switch 12 to deenergize the motor M1. Since the valve Vis in open position there is maximum generation of heat in the furnaceF.

It is to be further understood that upon the initial decrease in thetemperature of the furnace F, the voltage of thermocouple T alsodecreased and the mechanical relay MR functioned to rotate theslidewires i and 6 in a clockwise direction. Also the motor M rotatedthe slidewire 3 in a counter-clockwise direction a distancecorresponding with the movement of the valve V to its fully openposition. As the temperature of furnace F increases the voltage ofthermocouple T will rise and the mechanical relay MR will function torebalance the measuring circuit and the rising temperature will be shownon the record chart '!3 by virtue of the movement of recording pen l4,driven by a belt or violin string '15 through mechanical coupling 9extending to the mechanical relay MR. As soon as the mechanical relay MRdrives the slidewire I sufiiciently far in a counter-clockwise directionto balance the upper branch 24-1-25 of the network against the lowerbranch 293- 30, it will be understood that the relay R. will bedeenergized and its contact 32 will move to the open-circuit position.As the temperature 7, continues to ri h dewir l m i g lative to-c,,ntact 2 will unbalance the upper and lower branches of the network N inthe opposite direction to energize the relay R to complete a circuitthrough stationary contact 33 to energize the motor M in a valve-closingdirection. On initial closing movement of the valve V by the motor M thelimit switches 58 and T2 are closed. This operation occurs at the lowerlimit shown by the broken line 77 of Fig. 2 of the proportional band 78,the upper limit being shown by the broken line iii. The proportionalband or throttling range is defined as that temperature difference whichwill cause movement of the valve from one extreme limit to the other.

Inasmuch as the slidewire i has been rotated in a clockwise directionduring the. decrease in temperature of the furnace F, its displacementwill have unbalanced the network including the two uppermost branches'2-i--25 and 3.-38 for energization, by the unbalance voltage, of relayR1 to move the contact ll against contact 43. As long as the valve Vremained in its fully open position the circuit thro h contacts and 43remained open due to the operation of the limitswitch #2. Upon initialmovement or the valve V toward its closed position, as described, thelimit switch l2 closes to complete the circuit through contacts ll andto energize both the motor M1 to rotate slidewires 2e and as in theclockwise direction and to energize t e heater coil $2 to elevate thetemperature of l or 38. However, the motor it'll is ineifective i rotatethe slidewires in a clockwise direction Jeyond that point where theprojection did, hi against the abutment Eda and, therefo e, the motor M1rotates 1e clutch member while the clutch element remains station ry dueto the friction developed in th disc 55?). In this manner thedroop-correcting slidewires 29 and 3 3 may no be adjusted since theycannot be rotated in a dir ction to increase the resistance in the arml'2ii and to decrease the resistance in the arm li'l. However, thecircuit through the rheostat and the resistor 42 is effective and theresistance of s stor 38 is increased, the effect of which is in t" edirec tion to rebalance the networl: comprising the two upper branches.Accordingly, the heating resister 12 will remain energized untilrebalance between the two upper branches of network.

Returning now to the rotation of the motor it. in a direction toinitiate the closing movement of the valve V, it will be understood thatthe extent of the movement will depend upon the temperature of thefurnace F. Ordinarily, when the temperature arrives at the lower limitof the throttling range the heat generated in the furnace P will carrythe temperature above the lower limit so that the valve V will beoperated a substantial distance-toward its cl sed position. If thetemperature has been carried above the control point, the direction ofunbalance on the upper branches of the network will be reversed and therelay R1 will operate to close the circuit work is to produce unbalanceas between the upper and lower branches in the "direction for continuedoperation or" the motor Ivl in the valveclcsing direction. However, theincrease in the resistance of resistor 37 by reason of the elevation ofits temperature is in a direction to rebalanoe the upper two branches ofthe network for deenergization of the relay R1 to terminate thecorrective action being introduced by adjustment of slidewires 29 and35. The intermittent droop-corrector action continues until thetemperature of furnace F falls to the control point, shown by the solidline at in Fig. 2. For a given setting orpointer 5%, the temperature isshown in Fig. 2 by. the graph El as rising to the control point 89,overshooting the control point, and then gradually being decreased untilit arrives at the control point where it remains, the control actionthen bein effective to keep it there.

To prevent rise of the temperature of the furnace F above the controlpoint, the knurled operating knob 55 is rotated counter-clockwise; forexample, from its position at the point to a position say at the 30%point. With the knob at'the 50% point, the throttling range orproportioning band is equally divided above and below the control pointfor a temperature condition as shown in Fig. 2. With the knob moved tobring the pointer 5% to the 30% point, the throttling range will becomeeffective at a lower temperature, for conditions as shown in Fig. 3,with of the throttling range below the control point and 30% thereofabove. In this manner, overshooting may be prevented and the temperature8| will rise to the control point without over-shooting. On the otherhand, if the load demand or the quality of the fuel should change sothat the control point is reached in an undesirably long time, the knob55 may be rotated in a clockwise direction to shift the proportionalband upwardly so that the valve V will remain in its fully open positionuntil the temperature of the furnace F more nearly approaches thecontrol point. In the limit, with the pointer 54?) set at the valve willremain open until arrival of the temperature at the control point sincethe proportional band will be shifted until the whole of it is above thecontrol point.

Accordingly, it will be seen that the droopcorrector slidewires 2% and3E! perform dual functions. They compensate for droop and in conjunctionwith the mechanical limitation upon the extent of their adjustment inone direction, they exercise control over the maximum shift of thethrottling range with reference to the control point. Moreover, with thetemperature at the control point, pointer 53b in conjunction with thescale on the plate 58 indicates the position of the valve. Additionally,the setting of the pointer 54b predetermines the maximum open setting ofthe valve V when the temperature of the furnace first reaches thecontrol point.

Accordingly, it will be seen that the assembly, including the scaleplate 58 and the pointer 53b, yields valuable information as to theposition of the valve V at the control point and, thus, provides a readymeans of indicating the load demand on the system, and is in markedcontrast with prior systems in which it was necessary mentally to addtogether the readings from two scales in order to determine the positionof the valve with the temperature of the furnace at the control point.

The present invention has been particularly useful for batch processesthough, of course, it

is to be understood it can be used and is advantageous in continuousprocesses, particularly fol lowing periods of shut-down or disturbancesof the process. By including a manually operable circuit controllermovable from an automatic position A to a manual position M, Fig. 1, thecontrol of the valve V may be made directly dependent upon rotation ofthe knob 53. The movement of the operating handle 82 of the circuitcontroller from the automatic position A to the manual position M closescontacts 83 to complete a by-pass circuit around the slidewire I andopens contacts 84 in series with the supply line 28 and the contact 4|of relay R1. Accordingly, with the circuit controller in the manualposition, the slidewire l is effectively removed from the circuit andthe balance of the network between the upper and lower branches willdepend, of course, upon the resistance. values of resistors 24 and 25which have heretofore been assumed to be equal, and upon the relativepositions of slidewires 3, 29 and 38.

It will be remembered that slidewires 2'9 and 30 have already beenassumed to be equal and if it now be further assumed that each ofslidewires 3, 29 and 38 have equal maximum resistance values, anymovement of the slidewires 29 and 30 will require a correspondingmovement of slidewire 3 to rebalance the network. Accordingly, sinceslidewires 29 and 38 are operated directly by the knob 53, the valve Vwill be driven to readjust slidewire 3 to maintain the network inbalanced condition. The result of the foregoing is that the knob 53 willnot only indicate on the scale of the plate 58 the position of thevalve, but rotation of the knob 53 to a new position on the scale ofplate 58 will cause the valve to be moved to a position correspondingtherewith. It may be moved from one of its limits to the other.

If the knurled knob 55 be rotated in a clockwise direction until thepointer 54b is at a lower indication on the scale than the pointer 53bof knob 53, it will be apparent that under either manual or automaticcontrol it may be desirable to rotate the pointer 53b in acounter-clockwise direction. This may readily be done since theprojection 53a will press axially against the abutment 54a. As soon asthe projection 53a passes beyond the end of abutment 54a, the abutmentwill spring back into place, the position shown in Fig. 1, again toresume its function as an abutment to bring into efiect the assemblywhich provides frictional resistance against manual or motorclockwisemovement of the shaft 52 and knob 53 beyond the setting of thepointer 54b. I

With the above understanding of the invention, it is to be understoodthat when the valve V is in the fully closed position, the traveling nut66 will be moved to the right to open the limit switch 68a to interruptthe circuit to the motor M. At the same time the traveling nut 89 willbe moved into engagement with the link Ila to open the limit switch 12,thereby to disconnect from one side of the source of supply 28 the motorM1 and the selective circuits through contacts 48 and 43 of the heatingresistors 89 and 42. In Fig. 1, the invention has been shown somewhatdiagrammatically though with many of the features of a commercialembodiment of the invention clearly illustrated}. A sectional elevationof such commercial embodiment is shown in Fig. 4 where it will beobserved the motor M1 drives through the worm 85 the worm wheel 5% whichalso forms the driving element of the clutch or friction drive 58. Thespring 5| takes the form of a plurality of spring fingers such as formedin warped discs which, by means of a washer 86, are pressed by nuts 81against the worm wheel 59b. The worm wheel 581) which is free to rotateon the shaft 52 is thereby pressed against a collar 88 secured to theshaft 52 as by a set screw. The worm wheel 59?) may be made of fibre orother suitable phenolic condensation product which has been quitesatisfactory for one element of the clutch or friction driving means.The spring also aids in the frictional drive by its reaction against thewasher 88 which is free to turn on the shaft and which is pressedagainst the flat face of the adjacent nut 81. The shaft 52 is supportedfrom a bracket 89 and from the housing 56.

The slidewires 29 and 39 have associated with them their contact fingersshown as cylindrical contact elements 98 and 9|. A bracket 92 is securedto the housing 56 and as shown the central part is cut away to provideclearance for the slidewires 29 and 38. To provide the extreme limitsfor the rotation of the slidewires 29 and 38, one stop member 93 extendsfrom the housing 56 into the path of the stop member 94 extending fromthe common disc or mounting carrying the slidewires 29 and 38.Connections to the ends of the slidewire are made by flexible conductorswhich may encircle a stationary bushing 95 through which the shaft 52extends, a fibre element 96 being provided to keep the conductors inpositions adjacent the disc of the slidewires. The connections are madeat a terminal board 98, the portion of the conductors adjacent theconnecting ends being held in place by a clamp 99. It will also beobserved from Fig. 4 that the knob '53 is secured to the end of theshaft 52 by a set screw. It is to be further observed that the bushing95 is rigidly secured to the bracket 89 and thus provides an elongatedbearing surface for the shaft 52.

As shown in Fig. 1, dial plate 58 has been provided with an opening 82aso that the operating handle 82 of the manually operable circuitcontroller may be positioned therein and thus be in a convenientlocation for the operator.

While the invention has been described as applicable to the control ofthe furnace F, it is to be understood, of course, the invention may beapplied to the control of any heat-changing means such, for example, asa cooling device, in which case suitable changes would be made in therelease controlling the cooling device in order to cause greater coolingwith increase of temperature and lesser cooling with decrease oftemperature, the necessary changes being well understood by thoseskilled in the art.

While the present invention has been described and claimed in connectionwith the use of variable resistances in the balanceable network l-2938-3for varying the action of the control system, it is intended that theterm resistance be generic in scope and include a resistance,

limiting the torque which may Ice-applied tothe shaft by said motor,circuit-adjusting means secured to said shaft, manual adjusting-meansfor said shaft for moving itagainst the frictional opposition oifered bysaid frictional device, a second frictional device supported,adiacentsaid manual adjusting means and including an adjustabie elementmovable relative tosaid manual adjusting means and engageable therebyfor producing africtional resistance against rotation of said shaftgreater than'the torque whichmay be transmitted through said firstfrictional device thereby to arrest motion oirsaid shaft at any selectedangular position thereof.

2. In a control system having a balanceable network for maintaining themagnitude .of a condition at a control point, the combination ofresistance means included in said network and adjustable in a directionto rebalance said network upon change in magnitude of said condition,droop-correcting means included in said network to correct for adrooping characteristic of the control systei means for adjusting saiddroopcorrecting means during unbalance of a portion of said network, andmeans for variably limiting in one direction the extent to whichlsaiddroopcorrecting means may be adjusted comprising a frictional drivebetween said droop-correcting means and its driving means and anadjustable abutment engageable by an element drivenwith saiddrocp-correcting means for preventing fur-- ther adjustment thereof insaid one direction.

3. In a control system having a balanced-isle network for maintainingthe magnitude of a condition at a control point, the combination ofresistance means included in said network and adjustable in a directionto rebalance said network upon change in magnitude of said condition,(troop-correcting means included in said network and adjustable forcorrection of a drooping characteristic of the control system, drivingmeans for adjusting said (troop-correcting means actuated duringunbalance of a portion ofsaid network, a friction device interposedbetween said driving means and said droop-correcting means, and meansfor variably limiting in one direction the extent to which saiddroop-correcting means may be adjusted comprising adjustable means forapplying to said droop-correcting means frictional opposition againstmovement greater than the torque which be transmitted by said frictiondevice.

4. In a control system having a halanceable network for maintaining themagnitude of a condition at a control point, the combination ofresistance means included in said network and adjustable in a directionto rebalance saidnet work uponchange in magnitude of said condition,(troop-correcting means included in said network to correct for adrooping characteristic of the control system, means including afriction driving device for adjusting'said droop-correcting means duringunbalance of a portion of said network, manual means for adjusting saiddroopcorrecting means against the opposition offered by said frictiondriving device, an adjustable stop engageable by an element driven withsaid droopcorrecting means, and means associated with said stop forintroducing opposition to movement greater than the torque which maybetransmitted by said friction driving device for variably limiting in onedirection the extent to which said droop-correctin'g means may beadjusted and providing for movement thereof upon reversal of adjustmentof said droop-correcting means.

Cir

12 5. Ina controlsystem the combination with a condition-changingcontrol element operable between predetermined limits to maintain themagmtude-ofa condition at a control point, of a balanceable networkincluding a variable impedance having an'element adjusted in response tochange in magnitude of the condition to be controlled, a second variableimpedance having an elementadjusted in accordance with adjustment ofsaid control element, said variable impedances establishing a;predetermined throttling range Withrespect to the control point,droopcorrecting means included in said network to correct for a droopingcharacteristic of the col .trol :systema second balanceable networkincludingrsai'dfirst named variable impedance for defining saidccontrohpoint, means responsive to unbalance of said :second network foradjusting said droopecorrecting means to correct for said droopingQha-racteristic, and means associated with said. droopcorrecting meansfor shifting the throttling range ;so that it will be divided in anydesired ratioabove and below the control point comprisingcooperatingstructures one of which is :drivenby said adjusting means for variablylimitingdn one direction only theextent to which saiddroop-correctingmeans may :be adjusted.

6.1In .acontrol system havinga balanceable network for maintaining themagnitude of a condition at a control; point, the combination ofresistance means included in said network and adjustable in-a directionto rebalance said network-upon change in magnitude of said condition,droop-correcting-means included in said network andadjustableforcorrcection of a drooping characteristic of :the control system,driving means for adjusting gsaid droop-correcting means, a clutchdevice interposedbetweensaid driving means and said droop-correctingmeans, and means for variably limiting in one direction the extent towhich said .droop-correcting means may be adjusted comprising adjustablemechanical means for applying to said droop-correcting meansfrictionalopposition against movement greater than and ad ustable forcorrection of-a drooping characteristic of thecon-trolsystem, drivingmeans for adjusting =said idroopecorrecting means, a friction deviceinterposedbetween said driving means and said droop-correcting means,andmeans for variablylimitin in one direction theextent to which saiddroop-correcting means may be adjusted comprising a first pointer havinga projection thereon and driven with said droop-correcting means inresponse .to'adjustment thereof, a second pointer supported adjacentsaid first pointer for indicating thelimit to'which saiddroop-correcting means may be adjusted, said second pointer havingassociated therewith a mechanical stop zengageable with said projection,and frictional resisting means cooperating with said stop for applyingto'said 'droop-correctingmeans frictionalyopposition against movementgreater than the torque which may be transmitted by said frictiondevice.

8. A control system including droop-correcting means to correct for adrooping characteristic of 'said control system, means for adjustingsaid droop-correcting means, and means for variably limiting .in onedirection theex-tent to which said '13 droop-cor'recting means may beadjusted comprising a clutch member between said droop-corroot means andits driving means, a first pointer driven with said droop-correctingmeans for indicating the position thereof, a second pointer cooperatingwith a manually operable element both of which are positioned inconcentric relation with said first pointer, an indicating scalecooperating with said pointers, said element having a frictional meansfor resisting rotation of said element and said second pointer, and anabutment cooperating with said element and said second pointerengageable by a projection on said first pointer for preventing furtheradjustment of said droop-correcting means in said one direction.

9. A system for controlling the position of a control member adjustableto vary the supply of an agent affecting the magnitude of a conditioncomprising two balanceable networks unbalanced by change in magnitude ofsaid condition, one of said networks including droop-correctingresistances and a resistance mechanically coupled to said controlmember, means responsive to unbalance of the other of said networks forefiecting rebalance thereof and concurrently adjusting saiddroop-correcting resistances to shift the balance point of said one ofthe networks, means responsive to unbalance of said one of the networksfor adjusting said control member and said resistance coupled thereto,means preset to limit in one direction the extent of adjustment of saiddroop-correcting resistances, and switching means operated at apredetermined position of said control member to disable said firstresponsive means.

10. A system for controlling the position of a control member adjustableto vary the supply of an agent aifecting the magnitude of a conditioncomprising two balanceable networks unbalanced by change in magnitude ofsaid condition, one of said networks including droop-correctingresistances and a resistance mechanically coupled to said controlmember, and the other of said networks including temperature-sensitiveresistances, means responsive to unbalance of said other network forselectively heating said temperature-sensitive resistance andconcurrently adjusting said droop-correcting resistances, means presetto limit in one direction the extent of adjustment of saiddroop-correcting resistances, means responsive to unbalance of said oneof the networks for adjusting said control member and said resistancecoupled thereto, and switching means operated at a predeterminedposition of said control member to disable said first responsive means.

11. In a control system for maintaining the magnitude of a condition ata control point, a balanceable network unbalanced by change in magnitudeof said condition, resistance means included in said network, drivingmeans for adjusting said resistance means in direction to rebalance saidnetwork upon change in magnitude of said condition, droop-correctingmeans included in said network, means for adjusting saiddroop-correcting means between limits in direction further to unbalancesaid network to produce further adjustment of said resistance means, andan adjustable abutment for variably limiting in one direction the extentto which said droopcorrecting means may be adjusted by its said adjusting means from one of said limits toward the other of said limits,

12. In a control system for maintaining the magnitude of a condition ata control point, a balanceable network unbalanced by change in magnitudeof said condition, resistance means included in said network, means foradjusting said resistance means in direction to rebalance said networkupon change in magnitude of said condition, droop-correcting resistanceincluded in said network to correct for a drooping characteristic of thecontrol system, means for adjusting said droop-correcting resistance,and mechanical means preset to limit in one direction the extent ofadjustment of said droop-correcting resistance by its said adjustingmeans.

13. In a control system including a conditionresponsive element and anelement controlling the application of a condition-controlling agent,means producing actuation of said controlling element proportional toactuation of said responsive element, droop-correcting means foradjusting the relation of said controlling element with respect to saidresponsive element within predetermined fixed limits, means operable toadjust said droop-correcting means within said fixed limits, andmanually actuable limiting means operable to efiect an unsymmetricalshift in said fixed limits.

14. In a control system including a conditionresponsive element and anelement controlling the application of a condition-controlling agent,means producing actuation of said controlling element proportional toactuation of said responsive element, droop-correcting means foradjusting the relation of said controlling element with respect to saidresponsive lement within predetermined extreme limits defined by stopelements, means operable to adjust said droop-correcting means withinsaid predetermined extreme limits, and an adjustable stop membermanually adjustable between said extreme limits effectively to replaceone of said stop elements, the stop element defining the other of saidextreme limits remaining substantially stationary.

15. In a control system including a conditionresponsive element and anelement controlling the application of a condition-controlling agent,means producing actuation of said controlling element proportional toactuation of said responsive element, droop-correcting means foradjusting the relation of said controlling element with respect to saidresponsive element within predetermined extreme limits, means operableto adjust said droop-correcting means within said predetermined extremelimits, and a stop member adjustable between said predetermined extremelimits of operation of said droop-correcting means for arrestingoperation of said droop-correcting means at a predetermined adjustedposition of said controlling element relative to said responsive elementas said responsive element approaches a predetermined value of acontrolled condition.

16. In a control system for maintaining the value of a condition at apredetermined control point, the combination including aconditionresponsive element and an element controlling the applicationof a condition-controlling agent, means for producing actuation of saidcontrolling element proportional to actuation of said responsiveelement, droop-correcting means for adjusting the relation of saidcontrolling element with respect to said responsive element withinpredetermined extreme limits, means operable to adjust saiddroop-correcting means within said predetermined extreme limits, a stopmember adjustable between said predetermined extreme 15 limits ofoperation of said droop-correoting means. a scale associated with saidadjustable stop member, and index means actuated with saiddroop-correcting means, said index means cooperating with saidadjustable stop member and said scale for indicating a maximum adjustedposition of said controlling element when the controlled condition is atthe control point.

ELWOOD T. DAVIS.

WILLIAM CLARK, J R.

REFERENCES CITED The following references are of record in the file ofthis patent:

Number 16 UNITED STATES PATENTS Name Date Baak Sept. 19, 1939 Davis Nov.3, 1942 Lilja Dec. 26, 1944 Bristol Mar. 20, 1945 Isserstedt Feb. 18,1947

